Until recently displacement sensors such as microphones has been based on capacitor structures and impedance measurements. This has a number of disadvantages related to sensitivity, high voltage biasing, isolation between layers, alignment and positioning of membrane relatively to back electrode, high requirements to preamplifiers, and nonlinear response, all resulting in costly and complicated solutions.
In US2005/0018541 an improvement is described where a diffractive structure with modulated diffraction efficiency is used for providing an optical sensor element for measuring of displacement, pressure, acoustic signals or the like without requiring complicated optics. This is achieved by using a grating with focusing capabilities so as to remove or reduce the need for complicated optics. This is explained below on the basis of Fresnel zone plates. A Fresnel zone plate is known to provide a flat lens based on diffraction. This structure is illustrated in FIG. 1. The presented solution does however have a dynamic range being limited to approximately λ/8, as shown in FIG. 2, which shows the signal generated by moving the reflective surface. The signal is periodic (only the first period is shown in the figure). The two first working points are shown by the two circles at λ/8 and 3λ/8. The arrows shows the dynamic range of the sensor, where the generated signal is almost linear with the reflective surface position.
There are other types of position sensors producing sine or quasi-sine signals where the dynamic range is increased by combining several signals out-of-phase. Articles by Brown, David A., et al. “A symmetric 3×3 coupler based demodulator for fiber optic interferometric sensors.” SPIE, Fiber Optic and Laser Sensors IX Vol. 1584 (1991) [1] and Reid, Greg J., and David A. Brown. “Multiplex architecture for 3×3 coupler based fiber optic sensors.” SPIE, Distributed and Multiplexed Fiber Optic Sensors RI, Boston (1993) [2] discuss examples of fibre-based position sensors where it used 3 signals with 0, 120° and 240° phase shift to retrieve the position with a dynamic range of several wavelengths. It is also possible to use to signals in quadrature (90° phase offset), as described in Stowe, D., and Tsung-Yuan Hsu. “Demodulation of interferometric sensors using a fiber-optic passive quadrature demodulator.” Lightwave Technology, Journal of 1.3 (1983): 519-523 [3].
This is also mentioned in Optical interferometric sensor US 2009/0268211, especially in the abstract and paragraphs [0013], [0016] and [0020].
Another example of the prior art is shown in U.S. Pat. No. 7,355,720, which seems to be limited to Fabry-Perot resonators, where the amount of reflected light is measured. Light that is not reflected is transmitted. This principle is different from the grating readout of our invention, where a change in cavity length modulates the diffraction efficiency of the grating—i.e. how much light is directed into the 0-order (specular reflection) and −1/+1-orders mainly. If the membrane is totally reflecting, no light is transmitted. The use of a totally reflecting membrane in a Fabry-Perot device would not work: there would not be any modulation of the reflected light with change in cavity length.
However patent U.S. Pat. No. 7,355,720 mentions the use of several signals to extend the dynamic range, but first those signals are generated by different wavelengths. Then section [0020] mentions the use of a stack of multiple optical cavities that can be interrogated by several signals, also using multiple sources of light it seems. The article by D. Shin and B Kim, “A laser interferometer encoder with two micromachined gratings generating phase shifted quadrature”, Journal of Micromechanics and Microengineering. 21 (2011) 085039 illustrates an alternative displacement encoder using two different gratings with two different grating line periods. In this experiment, the signals from the two gratings are separated by the angle at which light is diffracted, which in turn is a function of the period of the grating lines. In order to separate the two signals, the device must be illuminated by collimated light (here a bulk HeNe laser with a condensing lens) and the photodetectors be placed relatively far away from the gratings (here 10 mm). The resulting measurement can be performed on a dynamic range of several times the illumination wavelength due to the two signals in quadrature, but the measurement will be influenced by variation in the laser intensity.
Thus it is an object of the present invention to provide a relatively simple and inexpensive displacement sensor having an increased dynamic range. It is also an object to provide a solution decreasing the effects of intensity variations in the light source. The objects of the invention are solved using a sensor according to the invention as described in the accompanying claims. The provided solution might also be integrated into a compact MEMS device, by using focusing diffractive patterns that focus light from a divergent light source (such as a VCSEL) onto different photodetectors, thus providing a compact and simple way of separating signals from the different diffraction patterns.
Signal processing is described is based on the abovementioned publications, especially US2005/0018541.